<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        div.outer{
            width:500px;
            height:200px;
            background-color: #ccc;
            margin:0 auto;
        }
        div.outer>div{
            width:33.33333333%;
            height:200px;
            float:left;
            background-color: red;
        }
        div.outer>div:first-child{
            background-color:blue;
        }
        div.outer>div:last-child{
            background-color:orange;
        }
    </style>
</head>
<body>
    <div class="outer">
        <div></div>
        <div></div>
        <div></div>
    </div>
</body>
</html>